1. Field of the Invention
The present invention relates to an optical device used in a transmitter for optical fiber communication, and in particular to an optical device including a Mach-Zehnder optical modulator for DQPSK (Differential Quadrature Phase Shift Keying).
2. Background Art
Mach-Zehnder optical modulators are being used in transmitters for optical fiber communication. In a Mach-Zehnder optical modulator, a flow of light is separated into two flows of light, which propagate through two different optical waveguides and are again combined into one. Phase modulation electrodes are provided on the optical waveguides. From the phase modulation electrodes, modulation voltages are applied across the optical waveguides to change the phases of light in the optical waveguides. To supply the modulation voltages to the phase modulation electrode, a feed line and a terminal line are respectively connected to opposite ends of each phase modulation electrode.
Digital signal modulation methods include differential quadrature phase shift keying (DQPSK) that assigns 2-bit data to each of four modulated phases. An optical modulator for DQPSK has two Mach-Zehnder optical modulators and therefore has four optical waveguides in total (see, for example, Japanese Patent Laid-Open No. 2009-229592).